(Prior Art 1)
In various fields of industries including the distribution industry, bar codes are widely utilized to control the physical distribution of commodities. The bar codes are also utilized having been printed on various cards such as, for example, pre-paid cards, commutation cards and data cards. These bar codes are read by an optical reader such as, for example, an optical scanner which subsequently processes information represented by the bar codes. Most bar codes carried by surfaces of commodities or cards are in the form of a pattern of stripes printed by the use of a black inking medium against a white background and visible to human eyes under visible rays of light. This visible mark is printed directly on merchandise or printed on a shaped sheet-like carrier which is in turn affixed to merchandise.
On the other hand, attempts have been made to form a mark such as a bar code by the use of a fluorescent substance capable of emitting an infrared region of light so that the fluorescent mark can be identified by an optical reader. While the fluorescent mark is generally invisible to the human eyes, the fluorescent mark emits a fluorescent light when the fluorescent substance contained therein is excited upon irradiation of an external light of a particular wavelength and, therefore, by analyzing the fluorescent light with an optical reader, information represented by the fluorescent mark can be decoded or identified. Even the fluorescent mark is, as is the case with the visible mark, printed directly on merchandise or printed on a shaped sheet-like carrier which is in turn affixed to merchandise.
As compared with the system in which change in intensity of light reflected from the visible mark is read in handling merchandise, a system for handling merchandise, including an optical reader for reading the fluorescent mark, has numerous advantages, some of which are listed below.
(1) Reading of the fluorescent mark is seldom affected adversely by the color of the merchandise and, therefore, the reliability in reading the fluorescent mark is high with the reading error minimized. PA1 (2) Even though the surface on which the fluorescent mark is formed becomes dirty, infrared rays of light emitted from the fluorescent mark has such a long wavelength that the reading error would seldom occur and the reliability is therefore high. PA1 (3) Since the fluorescent substance is substantially colorless under visible rays of light, printing of the fluorescent mark on the merchandise will bring no adverse effect on the aesthetic appearance of the merchandise. PA1 (4) Since the fluorescent substance is so invisible under visible rays of light that no one can recognize the presence of the fluorescent substance, it can provide security of information. PA1 Ln represents at least one element selected from the group consisting of Nd, Yb and Er; PA1 A represents at least one element selected from the group consisting of Y, La, Gd, Bi, Ce, Lu, In and Tb; and PA1 X represents a value within the range of 0.01 to 0.99. EQU DE.sub.1-X Ln.sub.X P.sub.Y O.sub.Z ( 2) PA1 D represents at least one element selected from the group consisting of Li, Na, K, Rb and Cs; PA1 E represents at least one element selected from the group consisting of Y, La, Gd, Bi, Ce, Lu, In and Tb; PA1 Ln represents at least one element selected from the group consisting of Nd, Yb and Er; PA1 X represents a value within the range of 0.01 to 0.99; PA1 Y represents a value within the range of 1 to 5; and PA1 Z represents a value within the range of 4 to 14. PA1 G represents at least one element selected from the group consisting of Y, Bi, Ce, Gd, Lu and La, and Er; and PA1 J represents at least one element selected from the group consisting of Sc, Ga, Al and In, and Fe. PA1 L represents at least one element selected from the group consisting of Y, Bi, Ce, Gd, Lu and La, and Yb; and PA1 M represents at least one element selected from the group consisting of Sc, Ga, Al and In. PA1 Q represents at least one element selected from the group consisting of Ca, Mg, Sr and Ba; PA1 R represents at least one element selected from the group Mo and W; PA1 X represents a value within the range of 0 to 1; PA1 Y represents a value greater than 0, but smaller than 1; and PA1 Z represents a value greater than 0, but smaller than 1. EQU (Nd.sub.1-X Yb.sub.X).sub.2Y Q.sub.8-3Y (RO.sub.4).sub.8 ( 10) PA1 Q represents at least one element selected from the group consisting of Ca, Mg, Sr and Ba; PA1 R represents at least one element selected from the group Mo and W; PA1 X represents a value within the range of 0 to 1; and PA1 Y represents a value greater than 0, but equal to or smaller than 8/3.
Particulars of interest in this connection are disclosed in, for example, the Japanese Patent Publications No. 55-33837, No. 60-29996 and No. 62-24024.
(Prior Art 2)
The fluorescent mark discussed above is formed by printing a fluorescent inking medium containing a fluorescent substance on a carrier such as, for example, a surface of the merchandise in a predetermined pattern. An infrared fluorescent inking medium has long been known and is disclosed in, for example, the U.S. Pat. No. 4,202,491. The infrared fluorescent inking medium disclosed therein is prepared from an inorganic fluorescent substance containing one or a mixture of neodymium (Nd), ytterbium (Yb) and erbium (Er). The inorganic fluorescent substance which utilizes Nd as an optically active element is known to emit a fluorescent light having a maximum intensity at about 1,050 nm in wavelength when irradiated with an exciting light of 800 nm emitted by a GaAlAs light emitting diode. The inorganic fluorescent substance containing a mixture of Nb and Yb as an optically active element is known to emit a fluorescent light having a maximum intensity at about 980 nm in wavelength when irradiated with an exciting light of 800 nm emitted by a GaAlAs light emitting diode. Similarly, the inorganic fluorescent substance containing a mixture of Yb and Er as an optically active element is known to emit a fluorescent light having a maximum intensity at about 1,050 nm in wavelength when irradiated with an exciting light of 940 nm emitted by a GaAs light emitting diode, and the inorganic fluorescent substance containing a mixture of Nd, Yb and Er as an optically active element is known to emit a fluorescent light having a maximum intensity at about 1,050 nm in wavelength when irradiated with an exciting light of 800 nm emitted by a GaAlAs light emitting diode.
(Prior Art 3)
The fluorescent substance disclosed in, for example, the Japanese Patent Publication No. 56-4598 makes use as the optically active element of Nd having a high absorption characteristic with respect to the infrared region of light, in combination with a fluorescent material capable of exhibiting a high intensity of light emission such as, for example, an alkaline metal salt (for example, Na.sub.2 MoO.sub.4 or the like) which is material for the matrix having a high efficiency of transmission of exciting energies from the optically active element to the emission center, or Yb having an emission center capable of favorably matching in wavelength with a Si photodetector.
(Prior Art 4)
For example, the Japanese Patent Publications No. 54-22326 and No. 61-18231 disclose a method of detecting the presence or absence of the fluorescent mark. In this known method, the fluorescent mark is prepared by the use of a fluorescent substance which emits a fluorescent light when irradiated with an exciting light within the infrared region of wavelength. This known method utilizes the difference between the center wavelength of the exciting light projected onto the fluorescent mark and that of the fluorescent light emitted from the fluorescent substance as a result of the irradiation of the exciting light and, for this purpose, only the fluorescent light is separated by an optical filter from rays of light reflected from the fluorescent mark so that the presence or absence of the fluorescent mark can be eventually detected.
The applicant has suggested a method of and an apparatus for detecting the position of a fluorescent mark by intermittently irradiating the fluorescent marking with the exciting light and then detecting the presence or absence of afterglow emitted from the fluorescent marking during the intermission of irradiation of the exciting light. (See, for example, the Japanese Laid-open Patent Publication No. 5-20512.)
(Prior Art 5)
FIG. 70 illustrates the prior art optical reading apparatus. The fluorescent mark shown therein is in the form of a fluorescent bar code 401 comprised of a pattern of parallel bars formed by printing the fluorescent inking medium on a sheet-like carrier 404 such as, for example, a label. The fluorescent inking medium used to form the bar code 401 contains fluorescent microparticles dispersed and retained in a binder, said fluorescent microparticles being of a kind which emit, when excited by an exciting light of a particular wavelength, for example, infrared rays of light 402, a fluorescent light 403 of a wavelength different from that of the infrared rays of light 402.
An optical reading apparatus for reading information from the fluorescent bar code 401 includes a light emitter 405 for emitting the infrared rays of light 402, a light receiver 407 for detecting the fluorescent light 403 from the bar code 401 and rays of light 406 reflected from the carrier 404 and for converting them into an electric signal, an amplifier 408 for amplifying the electric signal and for outputting an analog reproduction signal, and a signal detector 409 for detecting from the analog reproduction signal of the amplifier 408 information represented by the bar code 401. The signal detector 409 used therein includes an analog-to-digital (A/D) converter which is operable to digitize the analog reproduction signal so that the information represented by the fluorescent bar code 401 can be reproduced.
For digitization of the analog reproduction signal, a comparator is generally utilized, having an input stage which is adapted to receive the analog reproduction signal A and a slice signal B of a predetermined level shown in FIG. 66 so that the analog reproduction signal A can be sliced by the slice signal B to provide a digitized signal.
(Problem 1)
In the various fluorescent substances and the various fluorescent marks formed by printing the fluorescent inking media containing the respective fluorescent substances, both having hitherto been suggested, neither the relationship between the particle size of the particular fluorescent substance and the wavelength of the exciting light used nor the relationship between the particle size of the particular fluorescent substance and the wavelength of the fluorescent light emitted by such particular fluorescent substance has been taken into consideration. The conventional fluorescent substance has a particle size as relatively large as 5 to 6 .mu.m. On the other hand, for a light source for exciting the fluorescent substance, a semiconductor laser, for example, is generally utilized, capable of emitting a laser beam of about 0.8 .mu.m in wavelength while the fluorescent light emitted from the conventional fluorescent substance has a wavelength of about 1 .mu.m.
As discussed above, the conventional fluorescent particles have a relatively great particle size, i.e., a particle size as great as about 5 to 7.5 times the wavelength of any one of the exciting light and the fluorescent light. For this reason, if the fluorescent mar is prepared by the use of the fluorescent inking medium containing the fluorescent particles of that particle size, the fluorescent particles are deposited in such an overlapping relation that the exciting light projected towards a deposit of the fluorescent inking medium will not reach some of the fluorescent particles at a deep region of the deposit of the fluorescent inking medium, and for this reason, the efficiency of activation (excitation) of the fluorescent substance is lowered.
Even if some of the fluorescent particles at the deep region of the deposit of the fluorescent inking medium are excited to emit a fluorescent light, the fluorescent light so emitted tends to be partly intercepted by other fluorescent particles residing over such some of the fluorescent particles within the deposit of the inking medium, with the intensity of the fluorescent light consequently lowered. Consequently, the fluorescent light of such a low intensity often creates a problem associated with the reliability in detecting the presence or absence of the fluorescent mark.
Thus, partly because the efficiency of activation (excitation) of the fluorescent substance is low and partly because part of the fluorescent light excited will not emerges outwardly from an exterior surface of the deposit of the fluorescent inking medium and, hence, the intensity of the fluorescent light is consequently low, the prior art fluorescent substance poses a problem associated with the reliability in detecting the presence or absence of the fluorescent mark.
(Problem 2)
The fluorescent substance generally has such a property that when irradiated with the exciting light the fluorescent substance is activated to emit a fluorescent light in a progressively increasing quantity, but in the absence of the exciting light the quantity of the fluorescent light emitted decreases progressively. With the conventional fluorescent substance, the length of time, that is, the rise time, which passes from the start of irradiation of the exciting light upon the fluorescent substance and until the resultant fluorescent light attains a desired intensity is long. For this reason, a high velocity of movement of the fluorescent mark carrier relative to the optical reader cannot be employed, constituting an obstruction to the use of a high speed optical reader. If the relative velocity is increased, information represented by the fluorescent mark will no longer be read accurately and properly.
Although this is related to the relatively long rise time of the fluorescent light referred to above, the conventional fluorescent substance has a length of time (that is, the fall time) which passes from the interruption of irradiation of the exciting light upon the fluorescent substance and until the intensity of the fluorescent afterglow attains zero, that is, until the fluorescent light is no longer detected is long as well. For this reason, where the fluorescent mark consists of a plurality of parallel fluorescent bars, reduction in spacing between each neighboring fluorescent bars will render the light receiving element to detect a fluorescent afterglow emanating from the adjoining fluorescent bar, failing to provide an accurate information reading.
(Problem 3)
Inorganic powdery fluorescent pigments such as Nd, Yb and Er discussed hereinbefore have a relatively large particle size. Although this particle size would pose no problem if the fluorescent particles are tramped down with resin before use, the use of the fluorescent pigment of a relatively large particle size in an inking medium for use with an ink jet printer would, unless the particle size is reduced, result neither in a homogenous and beautiful print, nor in a high resolution during information reading. On the other hand, if the fluorescent particles are finely pulverized with the use of a mill, the fluorescent output would eventually decrease considerably.
The inventors of the present invention have also found that, in addition to the above discussed problems, the inorganic fluorescent pigments bring about an additional problem in that the response of the fluorescent substance to emit the fluorescent light subsequent to receipt of the exciting light is so low that a high speed reading is difficult to achieve.
(Problem 4)
In the Japanese Patent Publication No. 56-4598 referred to above, there is disclosed that the matrix material of the infrared-excitable fluorescent substance contains alkaline metal cations, Li.sup.+ Na.sup.+, if an anion thereof is chosen MoO.sub.2.sup.2- or Wo.sub.4.sup.2-. Since a salt of alkaline metal which is generally a monovalent metal has a relatively weak bond between the anion and the cation because of a small valence sufficient to be easily released to form a hydrate, the alkaline metal salt is water-soluble. Accordingly, the fluorescent substance prepared from the alkaline metal salt as a matrix material is extremely poor in water resistance to such an extent as to result in an obnoxious problem in practical use.
The infrared-excitable fluorescent substance is prepared by weighing, mixing and pressure-forming only the starting materials (for example, Na.sub.2 CO.sub.3, MoO.sub.3, Nd.sub.2 O.sub.3 and Yb.sub.2 O.sub.3), incinerating the preformed mixture and subsequently mechanically pulverizing it to provide the powdery fluorescent substance. In such case, the resultant fluorescent particles have a minimum particle size as small as about 5 .mu.m. Although this particle size permits the fluorescent particles to be used as a material for a printing ink medium such as used in, for example, a screen printing technique, the fluorescent particles of this particle size cannot be used as a material for an inking medium for use with an ink jet printer or for use in an inked ribbon. This is because the inking medium for use in the practice of a printing technique requires the fluorescent substance of 1 .mu.m or smaller in particle size, the fluorescent substance of about 5 .mu.m in particle size is not suited as a material for the inking medium that is used with the ink jet printer or in the inked ribbon.
The Japanese Laid-open Patent Publication No. 5-261 634 referred to above discloses that the fluorescent substance having its matrix material in the form of a salt of PO.sub.4 and activated by Nd and Yb can be used in an inking medium for use in an offset printing technique provided that such fluorescent substance is pulverized to a particle size within the range of 0.1 to 3 .mu.m.
However, the infrared-excitable fluorescent substance of this system has been found that both of the rise time, required for it to emit a fluorescent light of the maximum intensity subsequent to irradiation of infrared rays of light, and the fall time required for the intensity of the fluorescent light to attain zero subsequent to interruption of the infrared irradiation are extremely long, and therefore, it cannot satisfactorily be used where the exciting light is in the form of a pulsating light of short duration and/or where a high speed reading with, for example, a high speed scanner is desired.
The inventors of the present invention, in an attempt to develop an infrared-excitable fluorescent substance having a high response, have examined the use of Na.sub.2 MoO.sub.4 as a matrix of the infrared-excitable fluorescent substance, but have found that, because Na.sub.2 MoO.sub.4 is water-soluble, the fluorescent substance having its matrix added with optically active elements has exhibited a poor water resistance. Also, the fluorescent substance obtained had a particle size greater than a few microns and have therefore been found not suited for use as a material for the ink jet printer or the inked ribbon or in a printing technique such as an offset printing process.
(Problem 5)
Hitherto, in preparing a fluorescent composition such as, for example, the inking medium for use with an ink jet printer and containing fluorescent particles, none of the particle size of the fluorescent substance used, the density of the fluorescent substance and/or the density of a binder used, and the relationship among viscosity, surface tension, specific resistance and pH value has not taken into consideration. For this reason, the fluorescent particles contained in the fluorescent composition have such problems that the fluorescent particles are apt to sediment in the fluorescent composition, exhibiting an unsatisfactory dispersion stability, that the fluorescent composition tends to run during the printing and/or that the fluorescent output is low.
(Problem 6)
In the prior art fluorescent inking medium containing the fluorescent particles, the fluorescent substance is employed in a quantity generally within the range of 30 to 85 wt % relative to the total weight of the inking medium and is in the form of an inorganic compound having a relatively large particle size as discussed hereinabove. The use of the fluorescent substance in such a large quantity brings about such a problem that a fluorescent ink deposit formed by printing the fluorescent inking medium is so raised as to provide a visible indication of the presence of the ink deposit. This is problematic in terms of security particularly when a fluorescent mark is desired to be formed by depositing the inking medium at a location where the ink deposit will not constitute any obstruction to the eyes.
(Problem 7)
With respect to the fluorescent ink deposit formed by the use of the conventional fluorescent inking medium containing the fluorescent particles, no surface roughness of the ink deposit has been studied. Since the fluorescent substance of the relatively large particle size as discussed above has been employed, the surface of the ink deposit is relatively rough, having minute surface irregularities. Irradiation of the exciting light upon the rough-surfaced ink deposit tends to result in scattering of the exciting light upon the surface of the ink deposit, accompanying reduction in quantity of the exciting light penetrating into the fluorescent ink deposit. Also, with the rough-surfaced ink deposit, the fluorescent light emitted internally from the fluorescent ink deposit is apt to scatter in all directions at the surface of the ink deposit and, therefore, the quantity of the fluorescent light received by a light receiving element may be reduced correspondingly.
Once the above discussed phenomenon occurs, an output generated from the light receiving element in response to detection of the fluorescent light emitted from the fluorescent ink deposit is so low as to bring about a problem associated with the reliability in detecting the presence or absence of the fluorescent mark.
(Problem 8)
An optical reading apparatus used in connection with the fluorescent mark is known and includes a semitransparent mirror disposed generally intermediate between the fluorescent mark to be detected and a photoelectric detector assembly inclusive of light emitting and receiving elements. The known optical reading apparatus is so structured that the exciting light emitted from the light emitting element may be projected through the semitransparent mirror onto the fluorescent mark carrier so that the fluorescent light emitted from the fluorescent mark on the carrier can pass through the same semitransparent mirror before it is detected by the light receiving element. With this structure, it has been observed that as the exciting light travels through the semitransparent mirror, generally half of the exciting light may be reflected in directions other than the direction towards the fluorescent mark carrier and/or that as the fluorescent light emitted from the fluorescent mark on the carrier travels through the semitransparent mirror, generally half o the fluorescent light may be reflected in directions other than the direction towards the light receiving element. For this reason, the quantity of the exciting light necessary to activate the fluorescent substance is in practice small and, hence, the quantity of the fluorescent light emitted is correspondingly small and, yet, the light receiving element receives the fluorescent light in a quantity generally half of the actually emitted fluorescent light. Therefore, the light receiving element issues a considerably low output, so low as to bring about a problem associated with the reliability in detecting the fluorescent mark.
(Problem 9)
In the prior art optical reading apparatus, the exciting light emitted from the light emitting element forms a round irradiating pattern of a size sufficient to encompass the size of the bar code forming the fluorescent mark. Irradiation of the round light spot upon the fluorescent mark does not affords a sufficient area of surface to be illuminated and the intensity of the fluorescent light emitted from the fluorescent mark is consequently low. If an attempt is made to increase the size of the round light spot to thereby increase the area of surface to be illuminated, the exciting light will encompass not only the bar code of interest, but also the bar code adjoining such bar code of interest. This in turn brings about reduction in S/N (signal-to-noise) ratio, accompanying a problem associated with the reliability in detecting the fluorescent mark.
(Problem 10)
In designing the prior art optical reading apparatus, neither the rise time of the fluorescent light subsequent to irradiation of the exciting light, nor the relationship between the width of a slit extending in a direction of transport of the fluorescent mark carrier and the speed of transport of the fluorescent mark carrier is taken into consideration.
Also, neither the fall time of the fluorescent light subsequent to interruption of the exciting light, nor the relationship between the interval between the neighboring bars of the fluorescent mark (i.e., the fluorescent bar code) and the speed of transport of the fluorescent mark carrier is taken into consideration.
Because of the foregoing, no information represented by the fluorescent ink deposit on the fluorescent mark carrier can be read.
(Problem 11)
In the prior art optical reading apparatus, a slit member is interposed between the sheet-like fluorescent mark carrier such as, for example, a paper, and an object lens assembly so that only that portion of the fluorescent light emitted from the fluorescent mark which is desired to be detected is received by the light receiving element through the slit in the slit member. Although this will bring about no problem if the fluorescent mark carrier has a substantially uniform thickness, there is a problem in that, if the fluorescent mark carrier having a relatively great, but irregular thickness is transported, the fluorescent mark carrier being transported may be blocked with its front engaged with the slit and/or the slit will be damaged.
(Problem 12)
The prior art method of detecting the mark such as disclosed in the Japanese Patent Publications No. 54-22326 and No. 61-18231 and the Japanese Laid-open Patent Publications No. 3-16369 and No. 5-20512 all referred to above, is such that the fluorescent light emitted from the fluorescent mark as a result of excitation by the exciting light is detected by a detector. However, the quantity of the fluorescent light incident on the detector considerably varied with change in environments and conditions in and under which the detection is performed and, therefore, in order to secure a high accuracy of detection, a complicated circuit processing is required, or the condition of use is limited.
As a result of studies conducted by the inventors of the present invention in view of the foregoing problems, it has been found that any one of the prior art detecting method is unable to properly and accurately monitor the change in detecting condition because of an insufficient quantity of information available other than data associated with the fluorescent mark.
(Problem 13)
In another prior art method in which an optical filter used to separate the exciting light, which has been reflected, and the fluorescent light from each other, since the wavelength of the emission center of the reflected exciting light and that of the fluorescent light are close to each other and the intensity of the fluorescent light is extremely low as compared with that of the reflected exciting light, the both cannot be properly separated from each other with no difficulty and, in a certain case, most of the reflected exciting light may remain unseparated and enter the light receiving element together with the fluorescent light. Once this occurs, the accuracy of detection is lowered.
(Problem 14)
According to the fluorescent mark detecting method disclosed in, for example, the Japanese Laid-open Patent Publications No. 3-16369 and No. 5-20512 referred to above, if external rays of light of a wavelength matching with or in the vicinity of the wavelength of the fluorescent light exist in the environment in which the fluorescent light is being detected, such rays of light may be sensed by and converted into an electric output by the light receiving element. This leads to generation of false information that, even though no fluorescent light has yet been emitted, the fluorescent light was detected. In order to avoid such false information, both of the site of emission of the exciting light and the site of detection of the fluorescent light are required to be shielded from the external light, thus limiting the environment in which the system is used.
(Problem 15)
In the optical reading apparatus of a structure shown in FIG. 71, both of the level and the amplitude of the analog reproduction signal discussed hereinbefore are considerably affected by physical surface properties of the carrier on which the bar code is formed in the form of the fluorescent mark. More specifically, where the surface of the carrier is made of material having a propensity of absorbing a transparent inking medium and also the exciting light projected from an illuminator unit, the quantity of the fluorescent light emitted from the fluorescent bar code and the quantity of light reflected from the carrier are so small that, as shown in FIG. 71(b), the analog reproduction signal exhibits a low level and a low amplitude.
Also, where the carrier is made of material having a propensity of absorbing the transparent inking medium and also that of reflecting the light projected by the illuminator unit, the analog reproduction signal is apt to offset towards a high level as shown in FIG. 71(c). Moreover, where the carrier is made of material having a propensity of absorbing little transparent inking medium, but absorbing the light projected by the illuminator unit, the analog reproduction signal exhibits an increased amplitude as shown in FIG. 71(a).
Accordingly, with the prior art reading apparatus in which variation in waveform of the analog reproduction signal resulting from difference in type of the carriers on which the fluorescent bar codes are formed is taken into consideration, when the single reading apparatus is used to read one at a time the fluorescent bar codes formed on, for example, the respective carriers made of different materials, or when the reading apparatus is used to read one at a time the fluorescent bar codes formed on different portions of the single carrier which are made of varying materials, a problem is apt to occur in that the bar codes will not be accurately read. This problem may be avoided if an optical filter (a single wavelength filter) operable to cut off the entire light reflected from the carrier is disposed in front of the light receiving element. However, the single wavelength filter is expensive and cannot, in terms of cost, be installed in the optical reading apparatus and, instead, the above discussed problem does often occur since a band-pass filter operable to cut off a portion of the reflected light is generally employed.
Accordingly, a primary object of the present invention is to provide a highly reliable fluorescent substance having a high emissive output, a fluorescent composition, a fluorescent mark carrier, an optical reading apparatus, a merchandise sorting apparatus and a merchandise sorting system all of which are effective to substantially eliminate the above discussed problems inherent in the prior art.
Another important object of the present invention is to provide the fluorescent substance and the fluorescent composition which are effective to substantially eliminate the above discussed problems inherent in the prior art, excellent in durability, fine in particle size suitable for use in various printing techniques such as those employing an ink jet printer or an inked ribbon, and capable of exhibiting a high response.
A further important object of the present invention is to provide an optically detectable mark effective to substantially eliminate the above discussed problems inherent in the prior art and with which change in environmental conditions in which data are being detected can be readily and properly determined.
A still further important object of the present invention is to provide a detecting method and an optical reading apparatus effective to substantially eliminate the above discussed problems inherent in the prior art and capable of accomplishing accurate detection of the position at which the mark is formed, regardless of deterioration in condition in which the fluorescent light emitted from the mark is detected.
A yet further important object of the present invention is to provide the optical reading apparatus effective to substantially eliminate the above discussed problems inherent in the prior art and capable of accomplishing assured detection of the fluorescent mark without being adversely affected by environments in which it is used.
A different important object of the present invention is to provide the reading apparatus effective to substantially eliminate the above discussed problems inherent in the prior art and capable of accurately reading the fluorescent light at all times with no need to use the expensive single wavelength filter and regardless of the waveform of the analog reproduction signal.